To apologise, the phone and broadband provider offered their customers a free upgrade, with options including free Sky Sports for three months or a SIM card with a 100 minute, 250 text and 250MB monthly allowance free for 12 months.

The deadline to choose an upgrade was initially December 31st, but was extended to January 7th by TalkTalk on December 29th.

TalkTalk updated their website, individuals' personal online accounts and the customer service recorded message to notify customers. But it stopped short of directly emailing or writing to them.

#talktalk free upgrade is a joke, I didn't choose to stay I was forced to by not being allowed to break contract. You can't buy back loyalty

Separately, many customers have struggled to claim their upgrade online and have had difficulty contacting the company by phone due to lengthy waiting times.

One customer told Telegraph Money that it was not possible to apply online, and when he tried to contact customer services on the 31st he was wrongly informed that there was a team handling the upgrades who had gone home at 3pm.

He was surprised to hear that the deadline had been extended, as he was not notified by TalkTalk.

#TalkTalk will contact ombudsman about free upgrade.Not available to all customers only those with direct debit #DidoHarding

A TalkTalk spokesperson said: “The free upgrades are intended as a small gesture of thanks to our customers for their understanding during a challenging time, and of course we want as many of them to take it up as possible, which is why we extended the deadline.

"Information about that extension is, and has been, available on our website and phone lines, however we’re sorry that some customers who contacted us directly were given the wrong information.

"Customers can still select their free upgrade by logging into MyAccount or calling us on 0800 230 0403."